Reproduction Index Glossary

Maternal Recognition of Pregnancy

For most species, a critical need arises early in gestation for the mother to "recognize" that she is pregnant. More specifically, the concentration of progesterone in maternal blood must be sustained at a high level in order that the endometrium be maintained in a state conducive to embryonic survival. This means that the corpus luteum must not die and regress, as it normally does just prior to the onset of the next cycle.

Maternal recognition of pregnancy doesn't involve any type of conscious recognition by the mother. Rather, it is a process in which some type of signal prevents luteal regression, allowing the corpus luteum to persist and continue to secrete progesterone. This concept can be illustrated by looking at blood progesterone concentrations over time in a cycling sheep that becomes pregnant.

The pattern shown above for sheep is conceptually similar to what is seen other species with multiple cycles, including humans.

Although the end result is the same, several different mechanisms for maternal recognition of pregnancy have evolved in different groups of mammals. Some of this diversity can be appreciated by looking at humans, cows and dogs:

Blastocysts of humans and other primates secrete large quantities of a protein hormone called chorionic gonadotropin (CG), which is very similar to luteinizing hormone. CG binds to luteinizing hormone receptors in the corpus luteum and stimulates continued secretion of progesterone. It may also block signals in the corpus luteum that cause luteal regression.

In cattle and other ruminants, the corpus luteum regresses at the end of the non-pregnant cycle as a result of secretion by the endometrium of prostaglandin F2-alpha (PGF). The early ruminant embryo secretes copious quantities of a protein called interferon tau. Exposure of the endometrium to this hormone dampens the secretion of PGF, thereby blocking the signal for luteolysis. As a result, the corpus luteum survives and progesterone levels are maintained.

Dogs do not have multiple, sequential cycles like women or cows. Rather, they have a single cycle roughly every 4 to 6 months. Following ovulation, the pattern of progesterone secretion is essentially the same regardless of whether the bitch is pregnant or not. Consequently, dogs do not have a need for maternal recognition of pregnancy and apparently no mechanism for this process.

Several other interesting variations in maternal recognition of pregnancy have been characterized among mammals. The common theme is that the early conceptus is the source of the signal that interferes with normal luteal regression at the end of the cycle. This makes good biological sense - in essence, the embryo is shouting out its presence to the corpus luteum, saying please do not regress, I need your support!

A final important point should be made. The window of time for maternal recognition of pregnancy to occur is narrow, and failures either in sending or receiving the signal may well be a significant cause of early embryonic death. If, for example, a human embryo fails to secrete adequate amounts of CG in time to rescue its mother's corpus luteum, it will die. Luckily, you didn't have this problem, or you wouldn't be reading this page!

Index of: Fertilization and Early Embryonic Development
Cleavage and Blastocyst Formation Introduction and Index

Last updated on June 10, 2000
Author: R. Bowen
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